1. Field of the Invention
The present invention relates to a technique for supplying a common voltage of a liquid crystal display (LCD) and, more particularly, to a common voltage driving circuit of an LCD capable of preventing a common voltage from being floated in a liquid crystal panel.
2. Description of the Related Art
In line with the recent development of information technology (IT), the demands for flat panel display devices are rapidly increasing. An LCD is a typical flat panel display devices.
The LCD is a display device in which image information is individually provided to pixels arranged in a matrix form to control light transmittance of the pixels to thus display a desired image. For this purpose, the LCD includes a liquid crystal panel on which pixels, the minimum units for implementing an image, are arranged in a matrix form and a driving IC (Integrated Circuit) (driver) for driving the liquid crystal panel. In addition, the LCD includes a backlight unit that provides light, because the LCD does not emit itself.
In general, if the liquid crystal panel includes a common voltage driving circuit (drive IC), a positive polarity or negative polarity common voltage is applied to the liquid crystal panel via the common voltage driving circuit. In this case, the common voltage of a desired level (intended level) cannot be stably provided due to a parasitic capacitance or a leakage current present near the driving circuit or peripheral circuits.
For example, when a negative polarity common voltage is supplied to the liquid crystal panel via the common voltage driving circuit, voltage of an output node (Q-node) does not maintain its initial level as intended but gradually changes due to a parasitic capacitance or a leakage current as shown in FIG. 1.
That is, voltages of the positive and negative polarity output nodes (Q-node, Q-Node) alternately maintain ‘Low’ level, but the voltages do not maintain its initial level (Ideal Case Q-node or Ideal Case Q-Node) as intended but rise gradually.
This results in the occurrence of a common voltage floating phenomenon, causing a defective screen image as shown in FIG. 2.
Thus, the related art LCD having the common voltage driving circuit cannot appropriately cope with the variation of the lower or higher common voltage level, causing degradation of picture quality.